1. Field of the Invention
The present invention relates to antennas; and more particularly, antennas used in wireless communication systems.
2. Description of Related Art
The rapid development of new wireless communication systems has created the need for a variety of new antenna configurations with a broad range of technical requirements. Common to many systems, for both voice and data, is frequency re-use via spatial division into cells, with a base station in every cell center. Cells are often further divided into sectors (typically 3, 4, 5 or 6 sectors per cell), each of which is served by an antenna.
A typical main beam of such a base station antenna must be fan shaped: narrow in the elevation plane to increase the power efficiency, and wide in the azimuth plane to cover one sector.
Some systems utilize polarization diversity to increase the effective signal to interference ratio, which means that the antenna is also required to be sensitive, independently, to two orthogonal polarizations. These could be horizontal and vertical (HP and VP), or slanted (.+-.45).
Many base station antennas are vertical linear arrays of microstrip patch radiators. It is known how to choose the vertical linear array parameters to provide control of the elevation beam width for both polarizations. Controlling the azimuth beam widths in two polarizations, however, is much more difficult, as there are few options available to a designer, especially in the case of a dual polarized antenna. In the case of a dual polarized antenna, the size of the radiating patch, which can provide some degree of control over the beam width, can not be changed at will as the size of the radiating patch is determined by the operating frequency of the antenna. Also, the radiating patch has to be square in order to operate at the same frequency in both polarizations. In many cases the size of the ground plane behind the antenna, which also provides a degree of control over beam width, can not be easily changed because of size limitations or other physical design requirements. Accordingly, a demand exists for a technique which can control the beam width of an antenna even when the size of the radiating element and the ground plane are fixed.